1. Field of the Invention
This invention relates to a dry etching method and, more particularly, to a dry etching method in which a dimensional loss may be prevented from being produced even if etching is carried out using a reverse tapered resist pattern as a mask.
2. Description of Related Art
In keeping pace with increased refinement of the design rule of semiconductor devices, such as VLSIs and ULSIs, researches in the field of photolithography are currently conducted with a view to improving the photoresist material as well as increasing the numerical aperture of exposure units or shortening the exposure wavelengths. Above all, excimer laser lithography employing an excimer laser light source, such as KrF excimer laser light (248 nm) in place of a conventional g-line (436 nm) or i-line (365 nm) of high pressure mercury lamps, has attracted attention as a technique of achieving high resolution with comparative ease.
Meanwhile, it is difficult in excimer laser lithography to directly use a novolac positive type photoresist, which has hitherto been customarily used in g-line exposure or i-line exposure. The reason is that, since the aromatic ring of the novolac base resin and the naphthoquinone diazide compounds used as a photosensitive material exhibits marked absorption in a wavelength range of the KrF excimer laser light, the photoresist is poor in sensitivity and in exposure light transmission properties so that the photoresist pattern becomes tapered in cross-section.
Thus a photoresist material capable of achieving high sensitivity and high resolution has been desired. Recently, a so-called chemical amplification resist has attracted attention as such photoresist. The chemical amplification resist is a photoresist of the type in which an acid catalyst is generated by photoreaction from a photoreactive acid catalyst generator, such as an onium salt or a polyhalide, referred to hereinafter as photo-acid generator, and in which heat treatment (post-baking) is carried out in the presence of the acid catalyst to proceed with a resist reaction, such as cross-linking or functional group conversion, thereby producing changes in the rate of dissolution.
The chemical amplification resist is classified into a positive type and a negative type, depending on the type of the resist reaction, and into a two-component system and a three-component system, depending on the number of basic components. Currently, a negative three-component type resist, employing novolac resin, DDT (p,p'-dichlorodiphenyl trichloroethane) and hexamethyrol melamine, as a basic resin, photo-acid generator and an acid crosslinking agent, respectively, is nearing the stage of practical utilization. Resolution achieved by this resist is based on a mechanism that the acid catalyst is generated from DDT by exposure by a KrF excimer laser, which acid catalyst promotes crosslinking of the base resin by the cross-linking agent during post-baking to render the light exposed portion alkali-insoluble.
However, the above mentioned negative type chemical amplification resist tends to be affected by exposure light volume distribution in a direction along the film thickness and becomes more difficult to dissolve on the surface and the near-by region of the exposed portion, so that the as-developed resist exhibits a characteristic reversed tapered cross-sectional profile. Such deterioration in the cross-sectional shape of the resist pattern may translate itself into dimensional loss, when etching is made with the resist pattern as a mask, depending on the particular etching mechanism employed.
It is now assumed that, as shown in FIG. 1a, a layer of a material 2, formed on a substrate 1, is etched, using a photoresist pattern 3 as a mask. The photoresist pattern 3 is not a rectangular pattern having an upright wall but is a pattern of a reversed taper having a pattern width d.sub.2 on the bottom surface of the pattern adjacent the layer 2, which width d.sub.2 is less than a pattern width d.sub.1 on the top surface of the pattern (d.sub.1 &gt;d.sub.2). The width d.sub.1 is the desired or targeted pattern width.
If the layer 2 is etched using such 8 photoresist pattern 3 as a mask, and the layer 2 is of a material that is etched mainly by an ion mode, such as silicon oxide, the pattern width of the as-etched layer 2 becomes equal to the pattern width d.sub.1, as shown in FIG. 1b. The reason is that, when etching a fine pattern mainly by the ion mode, the operating conditions of low gas pressure and high bias voltage are adopted, so that the mean free path of the ions i.sup.+ is extended and the incident direction is set so as to be substantially normal to the layer 2 being etched, with the result that the range of incidence of the ions i.sup.+ is regulated by the width d.sub.1 on the top surface of the photoresist pattern 3.
However, if the layer 2 is of a material that is etched mainly by the radical mode, such as, for example, polysilicon or aluminum-based material, the pattern width of the as-etched layer 2 becomes substantially equal to the pattern width d.sub.2. The reason is that, in distinction from ions, radicals r* cannot be aligned in their proceeding direction, so that etching proceeds from the bottom surface of the resist pattern 3 under the influences of the predominant obliquely incident components. The result is that a difference that represents a dimensional loss is produced between the targeted pattern width d.sub.1 and the produced pattern width d.sub.2.
In this manner, if the resist pattern has a reverse tapered cross-sectional profile, a dimensional loss is produced, depending on the type of the layer to be etched. However, such dimensional loss is not tolerated under the increasingly refined current design rule.